This invention relates to a sound source and measurement arrangement for seismic surveys, which arrangement is especially intended for use in a body of water, particularly in icy water. The sound source and measurement arrangement may also be used in shallow water, water with mud flats, muddy water and water with dense vegetation.
Sound source and measurement arrangements towed by a vessel are known for example from U.S. Pat. No. 5,144,588 and U.S. Pat. No. 5,185,726, according to which towed devices may be set to move at a desired depth by means of buoys or the like. Known sound source devices are shown in U.S. Pat. No. 5,018,115. The measurement devices may be towed by the same vessel as the sound source devices. Alternatively, the measurement devices may be towed by a different vessel from the sound source devices.
The known sound source arrays, especially composed of air guns, suitable for towing by a vessel are complicated and their handling during launching and retrieval is troublesome and requires special arrangements. The known devices are not suitable for use in ice-covered waters, because they are easily jammed among ice blocks and thereby damaged.
Further, such a known air gun usually has several cables, e.g. for supply of power, outside the external shell of the air gun. These cables are easily entangled in vegetation or ice blocks, or in other obstructions such as abandoned fishing gear, nets and wrecks.
U.S. Pat. No. 4,038,630 shows an arrangement with energy sources situated in a flexible tube and towed by a wire. Signals and energy are transmitted between the vessel and the equipment in the flexible tube through a separate transmission cable. The arrangement shown is not easily modified. The arrangement is said to be flexible so that it may be pulled out of the water over transport rolls into a vessel, but there is no reference to storage on a drum.
Depth control of devices towed in water by adjusting the buoyancy is known from GB 1,421,364 and U.S. Pat. No. 4,745,583, using elements with a volume that varies in response to changes in the amount of enclosed medium. Devices of this kind are not suitable for being reeled on a drum.
The assembly of a survey arrangement, i.e. the relative locations of the various functional components of the survey arrangement, must be easily changeable to suit different survey and maintenance situations. U.S. Pat. No. 4,631,711 shows an arrangement in which, at one side of a survey device, there is a recess for removably receiving a cartridge, which allows the cartridge to be exchanged. It is not easy to change the length of the device shown in U.S. Pat. No. 4,631,711.
For simple listening and measurement operations, devices mainly without projecting parts, so-called streamers, are known, as shown in U.S. Pat. No. 4,641,288. It is not possible to provide such a device with, for example, air guns, which require more space than is available in a conventional streamer.
It is an object of the invention to provide a sound source arrangement for survey purposes whose handling during launching and retrieval is easy and fast and is feasible also in bodies of water containing ice blocks or dense vegetation. In the event that there is a need to vary the equipment assembly, a modifiable structure is preferred. Another object is to provide a survey arrangement with a small cross-section allowing incorporation of all necessary means and devices, while maintaining sufficient flexibility that it is possible to store the arrangement reeled on a drum.